Requirements Modelling

University of Toronto Department of Computer Science

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Requirements Modelling



Lecture 11:Requirements Modelling

A little refresher: What are we modelling? Requirements; Systems; Systems Thinking

Role of Modelling in RE Why modelling is important Limitations of modelling

Brief overview of modelling languages

Modelling principles Abstraction Decomposition Projection Modularity



Refresher: Definitions

Some distinctions: Domain Properties - things in the application domain that are true

whether or not we ever build the proposed system Requirements - things in the application domain that we wish to be made

true by delivering the proposed system A specification - a description of the behaviours the program must have

in order to meet the requirements

Two correctness (verification) criteria: The Program running on a particular Computer satisfies the Specification The Specification, in the context of the given domain properties,

satisfies the requirements

Two completeness (validation) criteria: We discovered all the important requirements We discovered all the relevant domain properties

Application Domain Machine Domain

D - domain properties

R - requirements

C - computers

P - programs



Source: Adapted from Loucopoulos & Karakostas, 1995, p73

Subject System

Information system

Uses

builds

Maintains information

about

Needs information

about

contracts

Usage System

Development System

Refresher: Systems to model



Refresher: Systems Thinking



Modelling… Modelling can guide elicitation:

It can help you figure out what questions to ask It can help to surface hidden requirements

i.e. does it help you ask the right questions?

Modelling can provide a measure of progress: Completeness of the models -> completeness of the elicitation (?)

i.e. if we’ve filled in all the pieces of the models, are we done?

Modelling can help to uncover problems Inconsistency in the models can reveal interesting things…

e.g. conflicting or infeasible requirements e.g. confusion over terminology, scope, etc e.g. disagreements between stakeholders

Modelling can help us check our understanding Reason over the model to understand its consequences

Does it have the properties we expect? Animate the model to help us visualize/validate the requirements



Source: Adapted from Jackson, 1995, p120-122

For every B, at least one P exists such that R(P, B)

The application

domain

Designations for the application

domain

CommonProperties

The modelling domain

Designations for the model’s domain

B = BookP = PersonR = Wrote

Book: entityPerson: entity

author: relation

RE involves a lot of modelling

A model is more than just a description it has its own phenomena, and its own relationships among those phenomena.

The model is only useful if the model’s phenomena correspond in a systematic way to the phenomena of the domain being modelled.

Example:Book

title

author(0,n)

(1,n)name

ISBN

Person



“It’s only a model” There will always be:

phenomena in the model that are not present in the application domain phenomena in the application domain that are not in the model

A model is never perfect “If the map and the terrain disagree, believe the terrain” Perfecting the model is not always a good use of your time...

Source: Adapted from Jackson, 1995, p124-5

…every book has at least one author……every book has a

unique ISBN…

CommonPhenomena

…ghost writers……pseudonyms……anonymity…

…no two people born on same date with same

name…

Booktitle

author(0,n)

(1,n)name

ISBN

Person

DOB

Phenomena not captured in the model

Phenomena not true

in the world



Source: Adapted from Loucopoulos & Karakostas, 1995, p72-73

UML fits in here

Choice of modelling notation

natural language extremely expressive and flexible

useful for elicitation, and to annotate models for readability poor at capturing key relationships

semi-formal notation captures structure and some semantics can perform (some) reasoning, consistency checking, animation, etc.

E.g. diagrams, tables, structured English, etc. mostly visual - for rapid communication with a variety of stakeholders

formal notation precise semantics, extensive reasoning possible

Underlying mathematical model (e.g. set theory, FSMs, etc) very detailed models (may be more detailed than we need)

RE formalisms are for conceptual modelling, hence differ from most computer science formalisms



Desiderata for Modelling Notations

Implementation Independence

does not model data representation, internal organization, etc.

Abstraction extracts essential aspects

e.g. things not subject to frequent change

Formality unambiguous syntax rich semantic theory

Constructability can construct pieces of the

model to handle complexity and size

construction should facilitate communication

Ease of analysis ability to analyze for

ambiguity, incompleteness, inconsistency

Traceability ability to cross-reference

elements ability to link to design,

implementation, etc.

Executability can animate the model, to

compare it to reality

Minimality No redundancy of concepts in

the modelling schemei.e. no extraneous choices of how to represent something

Source: Adapted from Loucopoulos & Karakostas, 1995, p77



Survey of Modelling Techniques Modelling Enterprises

Goals & objectives Organizational structure Tasks & dependencies Agents, roles, intentionality

Modelling Information & Behaviour

Information Structure Behavioral views

Scenarios and Use Cases State machine models Information flow

Timing/Sequencing requirements

Modelling System Qualities (NFRs)

All the ‘ilities’: Usability, reliability, evolvability, safety,

security, performance, interoperability,…

Organization modelling:i*, SSM, ISACGoal modelling:KAOS, CREWS

Organization modelling:i*, SSM, ISACGoal modelling:KAOS, CREWS

Information modelling:E-R, Class DiagramsStructured Analysis:SADT, SSADM, JSDObject Oriented Analysis:OOA, OOSE, OMT, UMLFormal Methods:SCR, RSML, Z, Larch, VDM

Information modelling:E-R, Class DiagramsStructured Analysis:SADT, SSADM, JSDObject Oriented Analysis:OOA, OOSE, OMT, UMLFormal Methods:SCR, RSML, Z, Larch, VDM

Quality tradeoffs:QFD, win-win, AHP,Specific NFRs:Timed Petri nets (performance)Task models (usability)Probabilistic MTTF (reliability)

Quality tradeoffs:QFD, win-win, AHP,Specific NFRs:Timed Petri nets (performance)Task models (usability)Probabilistic MTTF (reliability)



the Unified Modelling Language (UML)

Third generation OO method Booch, Rumbaugh & Jacobson are principal authors

Still evolving Attempt to standardize the proliferation of OO variants

Is purely a notation No modelling method associated with it! Was intended as a design notation (some features unsuitable for RE)

Has become an industry standard But is primarily owned by IBM/Rational (who sell lots of UML tools

and services)

Has a standardized meta-model Use case diagrams Class diagrams Message sequence charts Activity diagrams State Diagrams Module Diagrams Platform diagrams



Meta-Modelling Can compare modelling schema using meta-models:

What phenomena does each scheme capture? What guidance is there for how to elaborate the models? What analysis can be performed on the models?

Example meta-model:

Goals

TasksAgents

own

refine

implement

refine

assigned to



Modelling principles

Facilitate Modification and Reuse Experienced analysts reuse their past experience

they reuse components (of the models they have built in the past) they reuse structure (of the models they have built in the past)

Smart analysts plan for the future they create components in their models that might be reusable they structure their models to make them easy to modify

Helpful ideas: Abstraction

strip away detail to concentrate on the important things Decomposition (Partitioning)

Partition a problem into independent pieces, to study separately Viewpoints (Projection)

Separate different concerns (views) and describe them separately Modularization

Choose structures that are stable over time, to localize change Patterns

Structure of a model that is known to occur in many different applications



Modelling Principle 1: Partitioning

Partitioning captures aggregation/part-of relationship

Example: goal is to develop a spacecraft partition the problem into parts:

guidance and navigation; data handling; command and control; environmental control; instrumentation; etc

Note: this is not a design, it is a problem decomposition actual design might have any number of components, with no relation

to these sub-problems However, the choice of problem decomposition will probably be reflected in the design



Source: Adapted from Davis, 1990, p48 and Loucopoulos & Karakostas, 1995, p78

based on symptoms: no response from device;

incorrect response; self-test failure;

etc...

based on location: instrumentation fault, communication fault,

processor fault, etc

Modelling Principle 2: Abstraction

Abstraction A way of finding similarities between concepts by ignoring some details

Focuses on the general/specific relationship between phenomena Classification groups entities with a similar role as members of a

single class Generalization expresses similarities between different classes in

an ‘is_a’ association

Example: requirement is to handle faults on the spacecraft might group different faults into fault classes



Modelling Principle 3: Projection

Projection: separates aspects of the model into multiple viewpoints

similar to projections used by architects for buildings

Example: Need to model the requirements for a spacecraft Model separately:

safety commandability fault tolerance timing and sequencing Etc…

Note: Projection and Partitioning are similar:

Partitioning defines a ‘part of’ relationship Projection defines a ‘view of’ relationship

Partitioning assumes a the parts are relatively independent

Source: Adapted from Davis, 1990, p48-51



A brief UML example

:patient

NameDate of Birthphysicianhistory

:in-patient

RoomBedTreatmentsfood prefs

:out-patient

Last visitnext visitprescriptions

:patient

NameDate of Birthphysicianhistory

:heart

Natural/artif.Orig/implantnormal bpm

:eyes

Natural/artif.Visioncolour

:kidney

Natural/artif.Orig/implantnumber

Source: Adapted from Davis, 1990, p67-68

1

0..1

0..21..2

0..10..1

Generalization (an abstraction hierarchy)

Aggregation(a partitioning hierarchy)



What is this a model of?



Summary

Modelling plays a central role in RE Allows us to study a problem systematically Allows us to test our understanding

Many choices for modelling notation In this course, we’ll use (and adapt) various UML notations

All models are inaccurate (to some extent) Use successive approximation …but know when to stop perfecting the model Every model is created for a purpose The purpose is not usually expressed in the model …So every model needs an explanation

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Requirements Modelling

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